As is well known, television receivers are susceptible to signal interference from various noise sources. This interference may arise from many different sources which operate to interfere with the television signal or with the processing of the television signal by the television receiver. Typical sources of such interference are automotive ignition systems and household motors. The term impulse noise interference is commonly used to describe such events, which can cause disruption to the automatic gain control (AGC) circuits and the synchronizing circuits, as well as to the video and chroma signals. As is well known, if the impulse noise is present in the video detector output signal during intervals it is not blanked, that noise can proceed through the video processing path and result in a noise image being developed on the screen of the picture tube. The impulse noise will also be supplied to the sync processing path and cause the sync separator to generate unwanted output signals. Because the horizontal sync from the sync separator is usually supplied to an AGC system in the receiver, the latter system may be disrupted by the noise-induced output of the sync separator.
These problems are well known in the prior art. U.S. patent application Ser. No. 07/897,812 filed Jun. 12, 1992, is incorporated herein by reference, particularly for its more detailed background description concerning the approaches taken in the prior art in attempting to overcome the disruptive effects of impulse noise interference on the automatic gain control (AGC) circuits, the synchronizing circuits, and the video and chroma signal processing circuits of a color television receiver.
AGC and sync circuits are limited-bandwidth systems and filtering has been used to render these circuits relatively immune to impulse noise. Video and chroma circuits cannot employ the filtering techniques that are used with the sync and AGC circuits because the impulse noise signals share the same frequency spectrum with the video and chroma signals. Accordingly, non-linear signal processing of some sort is often applied to such receivers, but this processing is usually not very effective.
High-performance television receivers often employ synchronous picture (pix) intermediate frequency (IF) demodulators. Synchronous demodulation may be done in two phases: an in-phase synchronous demodulation that detects the composite video signal and the accompanying modulated sound carrier, and a quadrature-phase synchronous demodulation that detects the chrominance signal and modulated sound carrier without much accompanying luminance information. The only baseband components in the quadrature-phase synchronous demodulator response are differentiated transients of sync pulses and luma.
Unlike envelope or peak detectors which invariably detect the impulse noise as black-going in a negatively modulated video carrier such as that used in the NTSC and PAL television broadcast standards, synchronous detectors demodulate the asynchronous impulse noise as alternately black-going and white-going noise. White-going impulse noise is particularly objectionable since it tends to bloom the picture tube. The amplitude-modulated video carrier is vestigial sideband, so the pix IF amplifier chain filtering is centered about 2 MHz away from the video carrier frequency. Ringing of this filtering by impulse noise generates a random-phase damped sinusoid of about 2 MHz frequency, usually of large amplitude, in the in-phase synchronous demodulator response. If a quadrature-phase synchronous demodulator is used, a random-phase damped sinusoid of comparable frequency and amplitude is also generated in the quadrature-phase synchronous demodulator response.
In U.S. Pat. No. 4,524,389 issued 18 Jun. 1985 and entitled "Synchronous Video Detector Using Phase-Locked Loop" Isobe et alii describe a television receiver having just an in-phase synchronous demodulator. Black-going impulse noise in the output signal of this video detector is detected by a black noise detector and is thereafter canceled to gray. The output signal of the black noise detector is supplied to a pulse-stretcher. The pulse stretcher output signal is used to control the cancellation to gray of white noise following the detected black noise. The Isobe et alii procedure has shortcomings. Likely as not, the initial signal swing of the synchronously detected impulse noise with significant energy will be white-going, rather than black-going. Each such a white-going initial swing undesirably causes an intense white spot in the picture. Collectively, these white spots are sometimes called "salt" noise in contradistinction to "pepper" noise, a term used to refer collectively to the black spots in the picture caused by inversion of impulse noises to black in a television receiver with a video detector of the envelope detector type. These white-going spikes in the video detector output signal also disrupt the chroma channel.
The inventor knows of previous techniques for suppressing white-going impulse noise in which the white-going impulse noise in the video detector output signal is sensed and subsequently replaced with black (or a prescribed gray) level to generate a modification of video detector response. The setting of the video noise inversion threshold in such systems is extremely critical. The depth of video modulation can vary considerably from one source to another; so, if the threshold for impulse noise detection is set too close to the white level, false tripping on high white level modulation will frequently occur. If there is a high chroma, due to standing waves or other antenna problems, the noise inverter will falsely trigger on the chroma signal. If the threshold level is too high, too much white-going impulse noise will get through and bloom the picture tube. The detected in-phase video signal generally changes to white before it is detected as white-going impulse noise, so the damage or interference to the picture is already apparent when action is instituted to suppress the white-going impulse noise in these previous techniques. Although the duty factor of the white-going impulse noise is reduced, the interference is still seen by a person viewing the televised picture.
As practiced in the prior art, the very act of noise inversion creates a high-slew-rate signal which propagates through the video and chroma channels of the television system or receiver. A black or gray streak is inserted in the video signal by noise inversion circuitry that responds to impulse noise to replace the noise with a prescribed video level, and this streak is readily evident on the screen when impulse noise occurs over an extended time. The chroma channel is shock excited by the large-amplitude, fast-rising noise inversion pulse; and the consequent ringing of the filters in the chroma channel causes chroma "twinkle". Chroma "twinkle" comprises color changes of short duration at the points in the television picture where impulse noise intermittently occurs. The color changes at each of which points reminds some viewers of the light emitted by a star, which is the reason the word "twinkle" is associated with this phenomenon.
The problem of the detected in-phase video signal changing to white before impulse noise is detected, so the damage or interference to the picture is already apparent when cancellation of impulse noise proceeds, is avoided in U.S. patent application Ser. No. 07/897,812 by detecting the impulse noise as it occurs in a video detector output signal and effecting noise cancellation in a delayed response to that or another video detector output signal. The television picture is subsequently derived from the delayed video signal after the noise cancellation. If the impulse noise is detected in the video signal supplied by an in-phase synchronous demodulator and is detected in only one sense, black-going or white-going, it is preferable to detect white-going impulse noise. The video detector output signal can be delayed a shorter time before effecting noise cancellation, while still avoiding blooming on white-going noise, thereby reducing hardware cost.
The problem of chroma "twinkle" is addressed in U.S. patent application Ser. No. 07/897,812 by using track-and-hold circuitry to effect noise cancellation in the delayed video detector output signal, thereby to avoid introducing a large-amplitude, fast-rising noise inversion pulse into that delayed signal, as would shock excite the chroma channel using that delayed signal for input signal. Effecting noise cancellation in a delayed response to video detector output signal in order to avoid white-spotting during the initial portions of impulse noise removes another source of shock excitation of the chroma channel using that delayed signal for input signal.
When standard NTSC television signals are received by a TV of the type which employs two pix IF demodulators, a first synchronous demodulator to detect an in-phase video signal which is designated as the I video signal, and a second synchronous demodulator to detect a quadrature signal designated as the Q video signal, the quadrature channel by its very nature has no luminance component and therefore has no low-frequency component. Accordingly, the Q video signal can have a wider dynamic range available for the higher-frequency components in the video signal. This is especially true in TV receivers where the I and Q video detector responses are digitized by analog-to-digital conversion circuitry.
The reduction of the low frequencies of baseband luma in the quadrature-phase video detector response comes about because of both upper and lower sidebands of the video carrier being available for these frequencies, so the low-frequency components of their respective heterodynes with the quadrature-phase video carrier tend to be equal in amplitude and thus to cancel, being opposite in polarity. The color subcarrier is further removed from the carrier and does not appear in the vestigial sideband, so the substantially single sideband character of the color subcarrier leads to its appearing in similar strength both in the in-phase video signal and in the quadrature-phase video signal.
Since there is no low-frequency component in the quadrature channel, impulse noise can be detected in both the positive- and negative-going directions using relatively low-amplitude threshold settings for noise detection, it is pointed out in U.S. patent application Ser. No. 07/897,812. There is no need to use a relatively high-amplitude threshold setting in one direction so as to be able to be non-responsive to the synchronizing pulses, since the quadrature channel has negligible response to the synchronizing pulses. Accordingly, when impulse noise is sensed primarily in the quadrature channel, the threshold of impulse noise detection can be set very close to the start of the impulse, both in amplitude and time. This is especially true when the impulse noise in the quadrature channel is sensed in both the positive- and negative-going directions. There is inherent immunity from lock-out provided by sensing noise in the quadrature channel detection, it is further pointed out in U.S. patent application Ser. No. 07/897,812. So a noise-protected signal generated in response to the sensing of impulse noise in the quadrature channel can be used also for generating automatic-gain-control (AGC) signals, thus reducing the circuitry required for protecting AGC from impulse noise, U.S. patent application Ser. No. 07/897,812 indicates.
The fact of there being no low-frequency component in the quadrature channel makes it attractive to use the video output of this channel as input signal to the chroma circuitry, the inventor points out. The chroma circuitry conventionally recovers first and second color-difference signals with a pair of its own synchronous demodulators, which synchronous color demodulators would be third and fourth synchronous demodulators in a television receivers having first and second synchronous demodulators for detecting in-phase and quadrature components of the video carrier. If the synchronous color demodulators are of simpler construction, so as not to provide detector response in which input signal as well as carrier is balanced out, recovery of the color difference signals from the in-phase video signal requires bandpass filtering before those synchronous color demodulators, to reject the low frequencies of baseband luma so they do not interfere with the color difference signals recovered by the synchronous color demodulators. The low frequencies of baseband luma are suppressed in the quadrature-phase video signal, substantially reducing the need for bandpass filtering before the synchronous color demodulators. Where the synchronous color demodulators are of a type providing detector response in which input signal as well as carrier is balanced out, bandpass filtering before the synchronous color demodulators will be unnecessary, even if balance against input signal is less than perfect.